This invention relates to improved well logging methods and apparatus, and more particularly relates to novel methods and apparatus for providing a plurality of functionally integrated subsurface measurements and for providing improved measurements and representations of acoustic energy transmitted through the borehole.
Since no one earth parameter, or even any one combination of such parameters, can of itself provide a definitive and conclusive indication of the presence of oil and gas in commercial quantities, there has been a continuing need to perform as many different types of logging measurements as possible. It is not uncommon for a particular measurement to provide data which cannot be clearly and reliably interpreted in the absence of other different but functionally correlative measurements which may, for example, have been taken at a different time or from a nearby well-site. This, in turn, has also contributed to the need to provide logging sondes and systems for generating a plurality of different logging measurements, whereby the array of such measurements can be correlatively recorded so that a data point from a particular logging measurement may be associated with a corresponding data point from another measurement generated at a correlative depth.
It has further become desirable not only to provide technology for storing logging data with correlative logging data obtained at different times or locations at the same depths, but also to provide a technique whereby such data may be generated and stored in a form such that it can be conveniently retrieved to provide information not easily obtained by mere comparative inspection of data obtained by conventional techniques, and whereby functional conclusions may be derived in a more accurate manner for judging whether to complete the well at a particular depth.
One such logging measurement which is desirable to generate in a form suitable for convenient merging with other data and retrieval for analysis is the electrical representation of acoustic energy oscillations from an acoustic logging tool as they traverse the borehole, known as the "acoustic signature."
In the past, it was conventional to record and process such signatures in analog form. However, many problems have been associated with such techniques, including noise susceptability, inaccuracies associated with well-known analog amplifier "drift" problems and the like.
These and other disadvantages are overcome with the present invention, however, wherein improved well logging methods and apparatus are provided for correlatively merging two or more sets of logging data which may then be employed in computerized analysis. Moreover, novel methods and apparatus of the present invention provide one particular logging measurement, namely the acoustic signature, in a digitized form which is particularly suited for convenient merger as well as for providing improved measurement of acoustic travel time.